Tony Abbott is hardly the first Liberal prime minister to face a serious backbench revolt on an issue but the depth of feeling in his own ranks against his paid parental leave scheme is still striking.

More typically revolts are over some specific item – for example a budget measure – but this one is notable because it contests a signature policy of the leader which he took to two elections.

Abbott, a convert to government-financed PPL, imposed his plan on the Coalition rather than putting it through the party room, and defends it passionately. Yet the internal critics are just as determined as he is, in trying to derail what they see as an expensive and inequitable policy that has little community support.

Coalition opponents include two core groups – very dry Liberals who regard the plan as middle class welfare, and pragmatic Nationals who believe their constituents have little to gain compared with higher income earners elsewhere. Conservatives are also concerned that the scheme, which Abbott sells as a work entitlement designed to boost labour market participation, disadvantages stay-at-home mothers.

A number of the “dry” critics remain silent but will be pleased with their colleagues’ attacks.

The Nationals have no reason to hold their tongues. They want to be heard by their supporters.

It must be beyond galling for Treasurer Joe Hockey to have to go out to defend the scheme, while he hears Nationals backbencher John Williams lecture on the need to be “conservative with the budget”.

In a folksy version of the message Hockey delivers on other issues, Williams declared: “I see no point in borrowing money to give to a young mum when the bub is going to have to pay it back with interest later in life.” Williams – a senator from NSW whose willingness to speak his mind puts him in the mould of Barnaby Joyce (in former days) – said his daughter-in-law had found the current scheme, brought in by Labor, very beneficial. He’s suggested a compromise that would extend the present scheme from 18 to 26 weeks and add in superannuation.

Hockey didn’t believe in the Abbott plan in opposition but has had to accept it (though at least budgetary circumstances forced the PM to reduce the proposed maximum payout from $75,000 to $50,000).

Questioned on the latest report about rebel Nationals and Liberals being prepared to cross the floor to defeat the legislation (not yet introduced), Hockey on Tuesday argued that the PPL plan was “a no brainer for regional Australia”.

It was “a massive win for farmers who don’t have paid parental leave schemes. Farmers are self-employed and for a lot of the mums in a farming household, they don’t get paid parental leave and now they are going to have replacement wages plus superannuation”.

Hockey was being loose with the facts. Farmers can get PPL under the scheme now operating, and the work test for the new scheme would be the same as for the present one.

This work test is very liberal. A person must have worked for at least 10 of the 13 months before the birth or adoption of their child, and worked for at least 330 hours in that 10-month period (just over one day a week) with no more than an eight-week gap between two consecutive working days.

Jenny Macklin, shadow minister for families, who brought in Labor’s scheme, says bluntly: “Joe Hockey is lying. Of course people who are self-employed, including farmers, are already eligible for Labor’s fair and affordable paid parental leave scheme. Anyone who fulfils the work test gets Labor’s paid parental leave scheme.” She accuses Hockey of trying “to trick” his colleagues.

The Abbott scheme would include superannuation payments, which the present one doesn’t, and the longer time period. And of course, higher paid women would get a bigger payout – the current scheme pays only the minimum wage. But few farming women would fall into that category.

While the backbenchers are huffing and puffing, it is possible some would pull back when the crunch came. It is serious matter to vote down the leader’s pet project and arms would be twisted in a big way to stop that happening.

But the huffing and puffing might itself be fatal for the plan. The government has considered the Greens its best chance to help get the plan through the Senate. The Greens’ policy has been to support such a scheme (in a less generous form than Abbott’s original). But they are now split, with some of them reluctant to back it. One condition they are talking about is that the government needs to show it has the support of its own ranks. If the Greens want a let out, signs of a revolt in Coalition backbench would clearly give it to them.

More generally Hockey, frustrated that so many of his budget measures are under threat, issued a warning to the Senate, as the new crossbenchers prepare to take their places in July. “If it just continually says no without any capacity to negotiate an improved outcome, then the Senate becomes irrelevant.

“It is just simply a road black and we either have to smash through that road block or the Australian people get the chance to change the government.”

A double dissolution in the foreseeable future would be bravery with a capital B.

Michelle Grattan does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.